imx-drm: Add mx6 hdmi transmitter support

Add mx6 hdmi transmitter support.

Original work has been done by Sascha Hauer and Tony Prisk.
Special thanks to Russell King for his carefully review, many bug fixes and
testing of the mx6 HDMI driver.

Tested on the following boards:
- mx6q sabresd
- mx6dl sabresd
- mx6solo wandboard

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>